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Home , Alula, Archaeopteryx, Avialae, Bird, Caudipteryx, Dromaeosauridae

THE

A. What is a ? Shared, derived characteristics of B. : significance, history, as bird, as C. birds D. Feathered E. Hypothesis1: birds are not dinosaur descendents - evidence Hypothesis 2: birds ARE dinosaur descendents - evidence

A. What is a bird? Living birds are diverse (~9,000 ) and they possess some derived characters that are analogous to significant derived characteristics in living (i.e., they evolved independently in birds and mammals). These include upright posture, high metabolic rate/warm bloodedness and large brain size.

All birds through time (beginning with the Late Archaeopteryx) belong to the Avialae; modern () birds belong to the clade Aves, also called Neornithes in some of the images from the lecture.

Shared derived traits that distinguish Aves from other living used to be considered the essential defining character of birds, but found in in the last decade show that that is not the case; birds are now defined by the suite of physical, physiological and skeletal specializations that apparently evolved to enhance their ability to . They include (but are not limited to):

Feathers: planar feathers with asymmetrically-positioned shafts used for (aka the ‘primaries’ on living birds). Feathers come in a variety of forms, including down, contour feathers, semi-plumes, symmetrical rectrices ( feathers) and remiges ( feathers). have the following components: , shaft, barbs and barbules, but not all feathers contain all of these elements. Feather architecture varies with function.

Endotherm (warm-bloodedness): necessary for prolonged flight

Four-chambered : necessary for endothermy (as in mammals)

Expanded with huge orbits: large brains and eyes that can light. Generally much better vision and perception than eyes. Heads supported by long, mobile necks with very complex muscle control.

1 Large brains: facilitate cogitation and sensory perception, including vision (see above), exceptional hearing abilities (especially for long- wavelength ), and atmospheric pressure detection.

Toothlessness: modern birds have no teeth

Carpometacarpus: reduced, fused wrist and , supports wingtips and to reduces flexion

Alula: feather on tip of hand that breaks turbulence over .

Shoulder joint: birds have a unique ‘pulley’ arrangement in their shoulders in which a attached to the supracoracoideus muscle passes through a hole formed by the intersection of the , and to attach to the upper surface of the humerus. Used for the upstroke during flight.

Enhanced skeletal rigidity: essential for flight control and maneuverability; it is apparent in the synsacrum, , and (reduced tail).

Furcula (wishbone): Flexible wishbone stores energy from the downstroke to assist with the upstroke.

Keeled : attachment point for the large flight muscles (pectoralis accomplishes the downstroke; the supracoracoideus the upstroke).

Three-and-one feet: three in front, one turned to the rear (as in most living birds) allows for perching, landing, , hanging, etc.

Pneumatic : air sacs in take the place of marrow and facilitate the unique, bellows-like avian .

Bladderless urinary tract: nothing is retained that is unnecessary and might inhibit flight

A key point of bird . The traits above did not evolve all at once. They accumulated over millions of and in many taxa leading from theropod

B. Archaeopteryx lithographica Significance: the oldest, uncontested bird discovered to date and a real ‘missing link’, with a combination of bird-like and dinosaur-like features. Perhaps the most famous single known.

History: First Archaeopteryx was a single feather, found in 1860.

2 The most famous Archaeopteryx is an almost complete skeleton that is fully feathered. Thomas Huxley studied Archaeopteryx and used them to argue the case for evolution; Darwin, oddly ignored Archaeopteryx in his multiple new editions of The Origin of Species that post-dated discovery of Archaeopteryx.

Only 7 /partial Archaeopteryx skeletons have been found, and all come from a fine-grained of age, exposed near the town of in Bavaria, . Archaeopteryx was about the size of a modern or raven. This limestone was deposited in a quiet coastal lagoon and contains well preserved remains of many (crabs, , shrimp, etc.).

Archaeopteryx possesses many bird-like characters, including: expanded braincase, asymmetric feathers, keeled sternum, pneumatic bones?, three-and-one foot with sharp (suggesting that it flew primarily and was not a ground-runner). Not clear if big- was in opposition to hand.

Archaeopteryx possesses many dinosaur-like characters, including: teeth, three fingered with claws, long tail stiffened with zygopophyses, a raptor-style saurischian , ascending process on astragalus. To be clear, a furcula that is not flexible is actually a theropod dinosaur trait.

C. Other Mesozoic birds The fossil record for all birds is quite poor because their bones are delicate. We have a record thanks to exceptional preservation a some select localities.

The early shows a gradual accumulation of flight- specialization characters in some lineages during the .

Rahonavis: the most unspecialized Mesozoic bird excepting Archaeopteryx, yet it is of age! This bird had a long tail and an enlarged 2nd toe (like a dromaeosaur). It may also have a pneumatic skeleton.

Post-Archaeopteryx bird evolution 1: : acquisition of a pygostyle ()

Post-Archaeopteryx bird evolution Stage 2: , flexible furcula, backstroke pulley in shoulder, reduced number of trunk vertebrae

3 Two Key Groups: : dominant birds of Mesozoic (e.g., ), a few shared derived characters we won’t worry about

Post-Archaeopteryx bird evolution Stage 3: Ornithurines: even shorter trunk, full caudal rotation of pubis, synsacrum, tarsometatarsus (e.g., Late Cretaceous (diver) and (-like)).

Alvarezesauridae: Odd flightless birds of the Late Cretaceous, including Shuuvia and Mononychus. We have no idea what to make of them.

Post-Archaeopteryx bird evolution Stage 4: Aves (Neornithes): loss of teeth

D. Feathered dinosaurs First discovered in the province of China in the mid-1990’s, in lake sedimentary rocks approximately 124 million years old ()

Include: (1997) - a little bipedal theropod with down along its back and many sharp teeth; one specimen was found with remains in its ribcage – a deinonychosaur with vaned feathers – a little sharp-toothed oviraptosaurian with symmetrically vaned feathers on its and tail, short arms, and Beipiasaurus – a large (-sized) with primitive feathers – a small theropod with feathers covering most of its body, including its arms and tail and fang-like teeth (2003) – small deinonychosaur with asymmetrically vaned feathers on its arms and legs

How do we know the feathered dinosaurs (except maybe Microraptor) didn’t fly? On most, feathers are symmetrically-vaned Relatively short arms (not long like birds) Relatively few feathers on arms and tail

So, what are feathers for if not flight? Insulation /species recognition -catching Increased speed/

4 E. Origin of birds Objections to birds as derived theropod dinosaurs: 1. Dinosaurs with feathers (Caudipteryx, Protarchaeopteryx) are really flightless birds that evolved dinosaurian characteristics 2. The first true feathers really appear in the Late , on the non- dinosaurian archosauromorph . 3. The first true bird () appeared in the . 4. Archaoepteryx (a bird) appears in the Late Jurassic, whereas dromaeosaurids (the supposed closest theropod relatives of birds) don’t appear in the fossil record until the Cretaceous. Current consensus: point 1 is wrong; Longisquama doesn’t have true feathers (point 2), Protoavis is too poorly known to be compelling evidence of anything (point 3), point 4 is true and a little troubling, but certainly doesn’t prove false a dinosaur ancestry for birds. After all, the oldest monotreme is also Cretaceous, but we believe its last shared ancestor with other mammals was .

Many shared derived characters of birds resemble those of dinosaurs: 1. Perforate acetabulum: SDC of Dinosauria 2. Opisthopubic pelvis: SDC of (some ; moreover, embryonic birds have a normal saurischian pelvis that modifies during development into a synsacrum 3. Thin-walled, hollow bones: SDC of 4. Ornithoid-style : SDC of Theropoda , , crocs, sauropods and ornithischians all have a distinctive eggshell microstructure that is different from the found in theropod and bird 5. Expanded braincases and kinetic skulls: SDC of Theropoda 6. Furcula/Wishbone: SDC of many Theropoda Many non-avian theropods have been discovered with furcula, including , , , , , , , Protarchaeopteryx, , etc. 7. Three-fingered hand: SDC of Theropoda: the chicks of the living have three-fingered hands that develop into a carpometacarpus as the chicks mature 8. Rigid : SDC of 9. Semilunate carpal, ascending process on astragalus: SDC of Semilunate carpals in birds allows for unique flexing of the wrist

5 10. Feathers: appears in Coelurosauria A wide variety of feather morphologies is found in coelurosaurs, but at present the proposed evolution of feathers is not reflected in the fossil record 11. Enlarged second toe claw (fossil ): SDC of Dromaeosauridae 12. Hand-arm structure: SDC of Dromaeosauridae Archaeopteryx had arms as long as its legs; this may be the culmination of a trend in the more derived coelurosaur theropods

In terms of evolutionary steps, it is much simpler to derive birds from dinosaurs than from any other group of organisms. Thus, most paleontologists consider Aves to be part of the Dinosauria and the tiny hellenae to be the smallest adult dinosaur.

F. Origin of Flight Bottom up: running and flapping, perhaps catching with . This later conjecture seems pretty random. Wrong motion. Climbing trees and gliding down: hard to get from there to flapping.

Recent work suggests that flapping helps birds run up inclines. A plausible route to flapping flight.

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